As will be known to those skilled in the art, lasers produce a narrow, high-intensity beam of monochromatic light and are used in a number of industrial, research and medical applications. Lasers consist generally of three fundamental components, i.e., a power supply, a resonator and a laser or "lasing" material. There are several classifications of lasers, such as free-electron lasers, dye lasers, and semiconductor lasers, with each type of laser being distinguished by its components and operating characteristics. The present invention relates to one of the most common types of lasers, solid-state lasers. In a solid-state laser, the laser material is either a crystal or a glass. Early solid-state lasers utilized a long-thin ruby crystal rod. As will be explained more fully, in order to create a composition which can be stimulated to emit electromagnetic radiation in the form of a laser beam, certain impurities or "dopants" are added to the laser material.
As with other types of lasers, solid-state lasers have a resonator cavity with internal reflective ends. A rod of laser material is positioned longitudinally within the resonator cavity between the end mirrors. A "pumping source" such as a xenon flashlamp is mounted in the resonator cavity and is used to optically pump the laser rod. Photons produced by the stimulated emission of the laser material along the axis between the mirrors are thereby reflected such that a concentrated beam of light is produced within the cavity. An aperture or other means is provided at one of the mirrors, known as the output mirror, which allows a fraction of the laser light to project out of the cavity in the form of a laser beam.
The process by which stimulated emission of light occurs is best described with reference to the energy states of the dopant ions. As stated, the laser material includes a host material such as a phosphate glass into which specific impurities are added. It is the presence of these impurities which give rise to laser activity. Generally, there is a primary laser dopant, the stimulated emission of which is responsible for the laser beam. There may also be auxiliary dopants which "sensitize" the laser glass. The dopants are typically added as oxides to the host material during preparation of the glass melt. The dopant ions have discrete or "quantized" energy levels. Each ion can be excited to move from a ground-state energy level to a higher or excited energy state. In many instances, several energy levels are suitable for a given ion. The dopant ions can move between these energy levels during laser operation by the absorption and emission of photons. Only those photons having precisely the amount of energy necessary to raise an ion from an initial level to a higher level or state are absorbed by the ion. In turn, an ion moving from a higher energy state to a lower state gives off a photon having the same characteristic energy. The release or emission of a photon by an excited ion can occur as a spontaneous decay to a lower energy state or by stimulated emission. Stimulated emission occurs when an ion in an excited state is struck by a photon of precisely the same energy as the photon which raised the ion into the excited state. Thus, it will be appreciated that the stimulating photon has the same energy as the photon which is released following stimulation. In this manner, two coherent photons exist following a stimulated emission.
In operation, the laser material is optically pumped by the flashlamp. Many of the dopant ions are then elevated to excited states from which they decay spontaneously as previously described. In order for stimulated emission to occur, a population inversion must be created by the flashlamp. Accordingly, the laser material is optically pumped such that the number of ions in excited states exceed the number of these ions at lower energy levels. When a population inversion exists, a spontaneous emission has a greater probability of stimulating the emission of a photon by another excited ion than being absorbed by an ion at a lower energy level.
In the confines of a resonator cavity, the ability of a spontaneous or stimulated emission of a photon to stimulate the emission of another photon is amplified by reflecting the photons between the mirrors at the ends of the resonator cavity. As the photons oscillate along the axis of the cavity, they repeatedly pass through the lasing material, causing the stimulated release of additional photons having the same energy. In this fashion, a narrow beam of coherent, monochromatic light is produced, a fraction of which is allowed to escape the resonator cavity as a laser beam.
It is known that the basic interaction of photons and ions which produce a laser beam can be augmented by the use of auxiliary dopants. More specifically, pump ions can be added to the host material along with the laser ion. Both the laser ion and the pump ion are optically pumped to higher energy levels by the optical pumping means. As will be appreciated, after a laser ion in an excited state emits a photon, either spontaneously or as a stimulated emission, it moves to a lower energy state. The laser ion can then be pumped back into the higher energy level by the flashlamp or by a photon emitted from the pump ion. Auxiliary dopants may interact synergistically among themselves and with the primary laser ion to produce lasers having higher efficiencies.
As previously indicated, a number of host materials are available for use in forming laser rods, including silicate and phosphate-based glasses. Due to its superior chemical durability and low laser threshold, phosphate laser glass has replaced silicate laser glass in many applications. Those skilled in the art will also appreciate that laser glass provides a number of advantages over the use of crystal laser media. Moreover, it is known that the success of a multi-dopant system in a laser crystal may not necessarily translate into a successful laser glass. Similarly, dopant systems are not per se interchangeable between silicate glass and phosphate glass.
In U.S. Pat. No. 4,770,811 to Myers, entitled "Sensitized Laser Glass," which is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference, a sensitized phosphate laser glass composition is disclosed in which the primary glass constituent is P.sub.2 O.sub.5. As described in the aforementioned patent, it was found that the addition of specified amounts of cerium and chromium to the glass as auxiliary dopants, along with either a neodymium or erbium primary dopant, improved the efficiency and sensitivity of the laser glass. In theory, the auxiliary dopants absorb energy in regions of the flashlamp emission spectrum away from the absorption bands of the primary dopant and then serve as pumping ions for the primary dopant.
In U.S. Pat. No. 3,533,956 to Snitzer, a laser material is disclosed which utilizes laser ions selected from the group of erbium, holium and thulium. Sensitizer ions are added which are selected from the group consisting of a ytterbium, erbium, neodymium, thulium, chromium and uranyal. It is stated that the system is useful in glasses, including phosphate glasses. In a paper by Thornton et al., entitled "Erbium Laser Technology," a sensitized YAG (yttrium-aluminum-garnet) crystal laser host material is disclosed which utilizes erbium, ytterbium and cerium as dopants. However, the absorption spectra of this material suggested that tetravalent cerium ions were produced during growth of the crystal, accounting for a lack of trivalent cerium ion absorption.
Phosphate-based laser glasses containing neodymium, ytterbium and erbium are also known. In a report by Woodcock, entitled "Multiple Doped Erbium Laser Materials," it is stated that a series of melts of zinc-aluminum-phosphate glasses were made which contained Y.sub.2 O.sub.3. It is also stated therein that in most instances these glasses also contain Nd.sub.2 O.sub.3. Additional sensitizing agents were generally present, such as CeO.sub.2, MnO, Cr.sub.2 O.sub.3, UO.sub.2, MoO.sub.3, PbO or Sb.sub.2 O.sub.3, and CeO.sub.2 was frequently added in combination with the latter oxides. Two of the melts described in the aforementioned paper included neodymium, ytterbium, cerium and chromium. The aforementioned paper further states that fluorescent lifetime measurements of the Yb.sup.3+ -ion indicated that quenching was taking place due to the presence of Cr.sup.3+ -ions.